The cogito ergo sum of Descartes is one of the best-known--and simplest--of all philosophical formulations, but ever since it was first propounded it has defied any formal accounting of its validity. How is it that so simple and important an argument has caused such difficulty and such philosophical controversy? In this pioneering work, Jerrold Katz argues that the problem with the cogito lies where it is least suspected--in a deficiency in the theory of language and logic that Cartesian scholars have brought to the study of the cogito. Katz contends that the laws of traditional logic have distorted Descartes's reasoning so that it no longer fits either Descartes's own account of the cogito in his writings or the role he assigns it in his project. Katz proposes that the cogito can be understood as an example of "analytic entailment," a concept in the philosophy of language whereby a statement can be a formally valid inference without depending on a law of logic. Developing and defending his thesis, he shows us that by grappling with an historical philosophical problem it is possible to make an original contribution to the advance of contemporary philosopy.